This is because users decided that they want a browser that spies on them.
At least in Germany in hacker and IT-affine circles, you will often be frowned upon if you voluntarily use Chrome or Edge (except if you have a really good reason).
That's largely the same here, at least for anyone worth their salt. But how does that matter when Mozilla's pulling things like this?
For years now your only browser choices are "Google", and "funded by Google", and it shows.
I can't even give someone too hard of a time for using vanilla chromium or similar anymore; Not like it's any worse than literally every other browser offering nowadays, minus rare exceptions like librewolf or ungoogled chromium that also add a whole host of minor technical complications to use.
It's not what Mozilla does, it's about what Mozilla says/claims.
Mozilla is a deceptive/defective entity here.
What happens when Alice is with Bill, but Bill is also abusive to a lesser extent? And "don't have a browser" is not an option.
So often arguments on this axis come down to how much convenience are you going to give up for the trust relation you desire. We get stuck if we mistake convenience for necessity thereby bringing absolutes into a continuous trade-off problem.
I'm thinking lately the myth of the "browser" and "web" as coherent data spaces is something even Sir Tim gave up on, right? If the centre cannot hold constellations of specialised clients (which are already "apps" in a sense) look like enduring in the near future at the expense of interoperability and standards. The "best browser" will be the one that strikes the most deals with the parts of the network people want to connect to. It's just like the best "game console".
That seems really bleak for the Internet qua people's network.
No doubt http/s and the worlds of port 80/443 will endure eternal, but the "Universal" search and information space the pioneers and then proto-Google aspired to now seems so remote that the idea of a "browser" is itself a little ridiculous to beards like me. I think today the "browser" has become a clique of PKI suites and CAs, at the behest of banking and retail, backed by broken but well meaning regulation, and unwittingly creating this monster we still call "The Browser". anyway, peace.
[0] I use w3m for 99% of my daily drive and a sandboxed degoogled chromium for any of the "messy stuff"
Derivative browsers don't really count here, as they depend on the upstream to not hurt them. For instance, if the parent project completely removes something essential for privacy, it it a lot of work to keep it in your code. The Manifest v2 removal is an example. Over time, when other changes are built on the removal, this creates an increasingly high burden. Eventually, the child project is starved. You simply do not want to be in this position.